The research objective of this project is to develop a reliable clinical test of utricular organ function that can characterize utricular organ impairment and localize the affected ear. The proposed study will determine the sensitivity of head-eccentric rotation protocols for detecting utricular organ dysfunction in humans with vestibular impairment. If proven effective, eccentric rotation testing could be rapidly adopted into clinical practice since only minor modifications to a conventional rotation chair are needed to provide eccentric rotation. The routine application of a test of utricular organ function in neurotology clinics would result in improved diagnosis of vestibular dysfunction and, therefore, would foster improved vestibular rehabilitation protocols. There are two specific aims of the proposed work. The first specific aim will characterize the eye movement responses evoked by utricular stimulation during head erect eccentric yaw rotation in normal subjects and subjects with vestibular impairment. The proposed experiments will employ a novel technique to produce high vergence angles that does not trigger ocular fixation mechanisms. As part of this specific aim, the interaction between utricular organ inputs and horizontal canal inputs will be studied in both normal (control) subjects and subjects with unilateral vestibular loss. The second specific aim will characterize the eye movement responses evoked by utricular stimulation during nose-down eccentric rotation in controls and subjects with unilateral vestibular loss. Studies will be performed using an earth-vertical axis rotation chair, and a video-oculography system to record eye movements. In addition, bitemporal horizontal electroculographic recordings will be used to determine whether this technique provides sufficient resolution for the proposed tests. If electroculographic recordings are sufficient, this would facilitate the adoption of the proposed tests into clinical practice. Recommendations for the use of eccentric rotation as a clinical utricular test will be provided as a result of the proposed experiments.